Recently, downsizing and weight reduction of mobile information terminals such as cell phones and notebook computers have been developed rapidly, and thus the batteries that function as the operating power supply are required to have a much higher capacity. In particular, nonaqueous electrolyte secondary batteries represented by a lithium ion secondary battery are widely used as the operating power supply for mobile information terminals because they have a high energy density and high capacity. Power consumption of such mobile information terminals tends to be further increased when accompanied with enhanced functions such as video playback and games. Thus, the nonaqueous electrolyte secondary batteries that function as the operating power supply are strongly required to have a much higher capacity and higher performance for extended playback, output power improvement, and the like.
In such a background, in order to further improve the capacity of the nonaqueous electrolyte secondary battery, various studies have been carried out. For example, lithium nickel oxide having a higher capacity is used in place of lithium cobalt oxide that is commonly used as a positive electrode material of the nonaqueous electrolyte secondary battery, and the final charge voltage of the battery is increased to provide a higher capacity.
However, as the nonaqueous electrolyte secondary battery has a higher capacity and higher energy density, there is a problem regarding reduced battery safety. Hence, recently, in order to improve the battery safety (suppression of an internal short circuit), techniques are disclosed for forming a porous inorganic particle layer on a surface of a positive electrode active material layer or a surface of a negative electrode active material layer.
For example, JP-A-2009-43641 discloses a negative electrode for a nonaqueous electrolyte battery in which a negative electrode active material layer containing a negative electrode active material and an aqueous binding agent for a negative electrode active material layer is formed on a surface of a negative electrode collector. In the negative electrode for a nonaqueous electrolyte battery, a porous inorganic particle layer containing inorganic particles and a nonaqueous binding agent for a porous layer is formed on a surface of the negative electrode active material layer, and the binding agent for a negative electrode active material layer includes carboxymethyl cellulose (CMC) having an etherification degree of 0.5 or more and 0.75 or less.
As disclosed in JP-A-2009-43641, the porous inorganic particle layer formed on a surface of an active material layer in a negative electrode for a nonaqueous electrolyte battery is formed by coating the surface of the active material layer with an inorganic particle slurry that is prepared by mixing particles of an inorganic oxide such as titanium oxide and aluminum oxide with a solvent and a binder.
However, particles of submicron size generally have the characteristic of ready agglutination depending on the shape or the composition ratio of solvents. When a slurry containing aggregates caused by the agglutinating property is coated on the surface of an active material layer, the coated face becomes uneven and a part of the surface of the active material layer is exposed. Consequently, the exposed portion of the active material layer causes a short circuit when foreign matter having high conductivity (for example, a piece of a collector) is present, and thus the porous inorganic particle layer cannot function as an insulating layer.
Furthermore, because the particles of submicron size have a high specific surface area, it is difficult to prepare a solution or slurry that is stable and with excellent process efficiency. Therefore, the problem exists that when a solution or slurry containing particles of submicron size is coated on an active material layer, a safety improvement effect cannot be sufficiently obtained because the porous inorganic particle layer is coated unevenly.